The present invention relates to a fuel property detecting system, and more-particularly to a system for detecting a property of gasoline employed in an internal combustion engine.
In order to improve performances of an internal combustion engine, various corrections as to fuel injection quantity have been executed. Japanese Patent Provisional Publication No. 6-101529 discloses a fuel injection-quantity correction method for improving an operational stability of an engine during a warm-up period. Generally, a part of fuel injected from an injector is attached on a wall surface of an intake port and flows along the surface into each cylinder. The quantity of this wall-flow fuel is largely affected by a fuel property, particularly, by a volatility of fuel. Therefore, in order to stably operate engines against different volatilities of gasoline, various correction values of a fuel injection quantity are generally matched with heavy-most gasoline having the lowest volatility in practically used gasoline.
However, under a condition that such correction values for fuel injection quantity are matched with heavy-most gasoline, if lighter gasoline is used, the various correction values generally becomes too large and tend to put the air-fuel ratio into a rich side and to degrade exhaust emission, particularly, CO and HC.
It is therefore an object of the present invention to provide a fuel property detecting system which estimates a fuel property of fuel in use in order to properly adapt the correction values of fuel injection quantity according to fuel in use.
A fuel property detecting system according to the present invention for an internal combustion engine and comprises a fuel injector, an air-fuel ratio sensor and a control unit. The fuel injector is installed to the engine and injects a quantity of fuel to the engine. The air-fuel ratio sensor is installed to an exhaust passage of the engine and detects an exhaust air-fuel ratio. The control unit is coupled to the fuel injector and the air-fuel ratio sensor. The control unit is arranged to calculate the quantity of fuel injected from the fuel injector according to an operating condition of the engine, to command the fuel injector to inject the calculated quantity of fuel, to sample data of a response wave-form of the exhaust air-fuel ratio in response to the injected quantity of fuel at a transient period, to identify a plant model as to fuel in use by controlling a parameter of a previously constructed plant model on the basis of the sampled data so as to decrease a prediction error between the plant model and a norm model, to calculate a cutoff frequency of the identified plant model, and to estimate a fuel property of the fuel in use from the calculated cutoff frequency of the identified plant model and data previous stored in the control unit.